The invention relates to sputter deposition of ionized material, such as ionized metal plasma sputter deposition, for manufacturing a semiconductor workpiece. More specifically, the invention relates to methods and apparatus for using a low frequency or DC transverse magnetic field to control the directivity of the ions of sputtered material deposited in openings in a workpiece.
Sputter deposition is a process commonly used to deposit films, typically of metal or metal compounds, on semiconductor workpieces during the fabrication of semiconductor devices. In sputter deposition, the semiconductor workpiece and a target are mounted within a vacuum chamber. A sputtering gas such as argon is flowed into the chamber while a DC power supply applies a negative voltage to the target relative to the electrically grounded metal walls of the chamber. The negative target voltage excites the argon gas near the target into a plasma state and accelerates argon ions from the plasma to bombard the target. The bombardment of the target causes an emission (i.e., sputtering) of atoms from the target surface. The sputtered atoms of target material leave the target with a distribution of angular trajectories so that the sputtered atoms deposit to varying degrees on all exposed surfaces in the chamber. The semiconductor workpiece is mounted close to, and parallel to, the target so that high proportion of the sputtered target atoms deposit on the workpiece.
In a non-ionized sputter deposition process, the plasma occupies a relatively small region near the target, and only a very small proportion of sputtered target atoms are ionized in the plasma. Because almost all of the sputtered atoms are neutral, any DC voltage applied to the semiconductor workpiece would have little effect on the angular trajectories of the sputtered atoms, so the workpiece typically is left electrically floating.
FIG. 1 illustrates an opening 15 (such as a via, plug, or trench) in a semiconductor wafer 16 being filled by sputtered target material 11 in a conventional non-ionized sputter deposition process. Because a high proportion of the sputtered atoms 10 of target material arrive at the wafer with highly non-perpendicular trajectory angles, an excessive amount of the sputtered target material will deposit on the upper part of the side walls of the openings, and an insufficient amount will deposit on the bottom and lower part of the side walls of the openings. Consequently, nonionized sputter deposition can be unsatisfactory for filling openings having high aspect ratio, i.e., a high ratio of depth to width.
An ionized sputter deposition process is the same as the non-ionized process just described, but it additionally employs an RF power source to excite a relatively high density plasma in a region between the target and the semiconductor workpiece. The RF-excited plasma ionizes a high proportion of the atoms sputtered off the target. A second DC power supply applies a negative voltage to the semiconductor workpiece relative to the electrically grounded metal walls of the chamber. The negative voltage on the workpiece relative to the plasma accelerates the ionized sputtered target atoms so that they impact the workpiece with more perpendicular trajectories than in non-ionized sputtering, which greatly increases the amount of material deposited at the bottom of high aspect ratio openings in the workpiece, as shown in FIG. 2.
Ionized sputtering can be a good deposition method when the objective is to completely fill the openings in the workpiece, but it can be too extreme a solution to the shortcomings of nonionized sputter deposition when the objective is to deposit only a thin wetting layer or barrier layer of uniform thickness on the side walls of the openings. Specifically, while non-ionized sputter deposition often deposits too high a proportion of the sputtered material on the upper walls of openings, ionized sputter deposition often deposits too high a proportion on the lower walls of openings, leaving inadequate coverage of the upper walls, as shown in FIG. 2.
A need exists for a sputter deposition apparatus and process that facilitates adjusting the proportion between deposition rates on the upper and lower walls of openings in the workpiece so as to improve the top-to-bottom uniformity of the thickness of the deposited material.
The invention is an ionized sputter deposition apparatus and method that employs a low frequency or DC transverse magnetic field to increase the transverse component of the trajectory of ions of sputtered material being deposited on the workpiece.
Adjusting the strength of the magnetic field will adjust the trajectory angles of the sputtered material being deposited on the workpiece, thereby controlling the ratio between the deposition rates on the upper and lower side walls of openings in the workpiece. Therefore, the invention permits optimizing the top-to-bottom uniformity of layers deposited on the side walls by adjusting the strength of the magnetic field.
The invention is especially useful for depositing thin wetting layers or side wall barrier layers having uniform thickness.